1. Field of the Invention
The present invention relates to power-on reset circuits for integrated circuit (IC) devices. More specifically, the present invention relates to tunable power-on reset circuits for dual-voltage integrated circuit devices.
2. Discussion of Related Art
Power-on reset circuits are used in integrated circuit (IC) devices to insure proper functionality of the IC device when power is initially applied to the IC device and to insure proper functionality of the IC device if power to the IC device is temporarily lost. Power-on reset circuits typically prevent logic circuits from functioning until after the power-on reset circuit determines that the applied supply voltage is adequate. For example, memory circuits in the IC device, such as flip flops or SRAM cells, may be asynchronously held to a definite logic level (typically logic low) until the supply voltage to the IC device is adequate. Typically, a power-on reset circuit detects when a supply voltage Vcc transitions from less than to greater than an adequate voltage Vad. For convenience, supply voltage terminals are given the same name as the supply voltage. Thus, supply voltage Vcc is driven on a supply voltage terminal Vcc. When supply voltage Vcc surpasses adequate voltage Vad, the power-on reset circuit enables the logic circuits of the IC device. For CMOS devices, a minimum Vad is typically the sum of the threshold voltage of an N-type transistor (Vtn) and the threshold voltage of a P-type transistor (Vtp). Most power-on reset circuits adjust adequate voltage Vad to be greater than Vtn+Vtp to provide a margin for error in supply voltage Vcc.
FIG. 1 shows a block diagram of a conventional power-on reset circuit 100. Voltage detection circuit 110, which is coupled between a supply voltage terminal Vcc and a ground terminal, detects when supply voltage Vcc is greater than adequate voltage Vad. If supply voltage Vcc is less than adequate voltage Vad, voltage detection circuit 110 drives a voltage detect signal VD on output terminal 111 of voltage detection circuit 110 to a power-off logic level (either logic low or logic high). If supply voltage Vcc is greater than adequate voltage Vad, voltage detection circuit 110 drives voltage detect signal VD on output terminal 111 of voltage detection circuit 110 to a power-on logic level (either logic low or logic high). Voltage detect signal VD of voltage detection circuit 110 is driven through a low pass filter 120 and a optional buffer circuit 130 to prevent spurious noise or ground bounces from accidentally causing a reset. In some embodiments, low pass filter 120 may invert the input signal and/or buffer circuit 130 may be an inverting buffer. Buffer circuit 130 outputs a power-on reset signal POR on output terminal 131. Power-on reset signal POR is distributed to the logic circuits of the IC device. Upon receiving an enabling power-on reset signal POR, the logic circuits can be set to a definite logic level or enabled to function. Some conventional power-on reset circuits are described by Lee in U.S. Pat. No. 5,394,104, entitled "Power-On Reset Circuit Including Dual Sense Amplifiers."
Due to the need for increased speed of IC devices, many IC devices use multiple supply voltage levels on a single integrated circuit device. As used herein, voltage supply circuits (not shown) provide a supply voltage on a supply voltage terminal. For clarity, the actual supply voltage on a supply voltage terminal and the supply voltage terminal are given the same reference name. Thus, for example "supply voltage Vcc1" refers to the actual voltage level found on "supply voltage terminal Vcc1". Each voltage supply circuit attempts to drive the supply voltage to a specific target voltage. The target voltage for the supply voltage circuit providing supply voltage Vcc1 is called the "Vcc1 target voltage."
Some microprocessors and programmable devices, such as field programmable gate arrays and programmable logic devices, use a first supply voltage Vcc1 for input/output logic circuits and a second supply voltage Vcc2 for internal logic circuits. Typically, transistors using lower supply voltages are able to switch logic levels at a faster rate. However, input/output logic circuits may require higher supply voltages to communicate to other IC devices on printed circuit boards. Thus, the Vcc1 target voltage for the input/output logic circuits is typically greater than the Vcc2 target voltage for the internal logic circuits. When a circuit is partly powered up, second supply voltage Vcc2 may be adequate, while first supply voltage Vcc1 may be inadequate. Under these conditions, the logic circuits which receive first supply voltage Vcc1 and provide signals to the logic circuits which receive second supply voltage Vcc2 would provide erroneous signals. Therefore, even the logic circuits which receive second supply voltage Vcc2 are likely to produce erroneous results. Thus, on IC devices with multiple supply voltages, there is a need for a power-on reset circuit to reset logic circuits which use a second supply voltage if a first supply voltage becomes inadequate.